The present invention relates to structural assemblies and, more particularly, relates to the joining of structural members by welding to form structural assemblies.
Conventional structural assemblies, as used in the manufacture of military and commercial aircraft and missiles, are commonly formed of lightweight, high strength materials such as aluminum, aluminum alloys, titanium and titanium alloys. These assemblies are commonly constructed using a bonded honeycomb-sandwich construction or a built-up structure from structural members that are fabricated using manufacturing methods such as a machined die-forging, investment casting or hogout machining from stock material. Conventional structural assemblies formed from these types of constructions generally include large numbers of parts and fasteners that can result in extensive tooling and increased labor costs during manufacture and assembly.
During use, aircraft structural assemblies are subjected to a variety of environmental conditions, temperature variations, severe acoustic and vibration environments, all of which create mechanical and thermal stresses. Over time, the application of cyclical stresses to bonded structural assemblies can lead to disbonding at the joints, and unless repaired, it can result in mechanical failure. Due to the large number of parts and fasteners utilized in the construction of conventional structural assemblies, maintenance and repair can be time consuming and labor intensive, which can be costly over the life of the assembly. The number of total parts utilized in a bonded honeycomb or built-up structure can also increase the overall weight of the aircraft. Consequently, conventional structural assemblies are generally costly to build and maintain and can adversely affect the weight of the aircraft.
In seeking better structural assembly designs, other types of joining methods have been proposed for assembling the component parts of the structural assemblies. For example, one such alternative joining method includes full penetration electron beam welding which produces an autogenous weld. Structural members joined by electron beam welding are fused together using the heat generated by a concentrated beam of high-velocity electrons impinging on the adjoining surfaces of the structural members. The kinetic energy of the electrons is converted into heat as the electrons strike the structural members. Electron beam welding is typically conducted under a high vacuum using an electron beam gun column to create and accelerate the beam of electrons, as is known in the art. The electron beam gun column generally includes an electron gun, which is comprised of an emitter, a bias electrode, and an anode, and ancillary components, such as beam alignment, focus and deflection coils. The high-energy density in the focused electron beam produces deep, narrow welds at high speeds, with minimum distortion and other deleterious heat effects to the structural members. For example, depth-to-width ratios for electron beam welds typically range between 10:1 and 30:1 with welding speeds as high as 200 mm/s (40 ft/min). Electron beam welds exhibit superior strength compared with welds formed using other fusion welding processes.
One technique of full penetration electron beam welding is referred to as the keyhole technique in which the electron beam creates a hole entirely through the structural members to be joined. The hole created by the electron beam is subsequently filled with molten metal as the beam moves along the interface defined by the adjacent structural members. Referring to FIG. 1A, there is illustrated one embodiment of the keyhole technique. As the electron beam 10 having a diameter D is moved along the interface or joint 12 between the structural members 14a, 14b, as indicated by the directional arrow 11, the molten metal is forced around the sides of the beam from the leading side 15a to the trailing side 15b where the metal solidifies to form the weld bead 16.
As illustrated in FIGS. 1A and 1B, due to spatter, bead fall through, and/or vaporization of the metal during welding or weld shrinkage upon cooling, there may be insufficient material to completely fill the keyhole as the electron beam 10 moves along the interface 12 between the structural members 14a, 14b, which can result in regions of underfill or undercut 18 within the solidified weld bead 16. The volume of underfill or undercut 18 increases when, as illustrated in FIG. 2, the electron beam 10 moves through a section of the structural members 14a, 14b where the thickness of the structural members decreases, as represented by txcex4. Since the volume of material on the leading side 15a of the electron beam 10 is less than the volume of material on the trailing side 15b of the beam, there is insufficient material to fill the keyhole as the electron beam moves through the structural members.
In order to compensate for any underfilling or undercutting during electron beam welding, conventional structural members 24a, 24b are typically fabricated with a raised portion 20a, 20b along the side of each structural member to be welded, as illustrated in FIG. 3. When the structural members are positioned adjacent to one another prior to welding, the raised portions 20a, 20b collectively form a weld land 20. As illustrated by the weld profile in FIG. 3, any underfilling or undercutting during electron beam welding of the structural members 24a, 24b occurs within the weld land 20 of the structural assembly 22. Once the structural members are joined together by the electron beam, the weld land 20 is removed from the structural assembly 22 using known mechanical machining processes, such as using cutting or grinding tools, to thereby provide a structural assembly having a smooth finished surface. While the weld land 20 prevents underfilling or undercutting within the weld bead 16 joining the structural members, the formation and removal of the weld land significantly increase the material, labor and tooling costs associated with the manufacture of the structural assembly.
As a result, there remains a need for an improved method of constructing structural assemblies, which minimizes the costs associated with manufacture and assembly of the structural assemblies, as well as reduces the overall weight of the aircraft. The structural assemblies must also be capable of providing high mechanical strength and structural rigidity.
The present invention provides a method of manufacturing a structural assembly including the steps of providing first and second structural members having preselected shapes and dimensions. The first structural member defines a first raised portion and the second structural member defines a second raised portion. According to one embodiment, the providing step includes forming the first and second structural members into preselected shapes and dimensions. The forming step can include casting, forging or machining the first and second structural members. According to another embodiment, the first and second structural members comprise plates, T-stiffeners or tubular members. According to still another embodiment, the first and second structural members are formed of aluminum, an aluminum alloy, titanium or a titanium alloy. The first raised portion of the first structural member is positioned adjacent to the second raised portion of the second structural member following the providing step to thereby define an interface therebetween and wherein the first and second raised portions define a substantially consumable weld land. Thereafter, the first and second structural members are irradiated with a high-energy source along the interface to remove surface irregularities that cause stress concentrations. The first high-energy source can include an electron beam or a laser. Advantageously, the consumed weld land does not require a post-weld mechanical machining step in order to provide a finished surface.
According to another embodiment, the method of manufacturing the structural assembly includes the steps of determining the shape and dimensions of a weld land based upon the shape and dimensions of first and second structural members such that the weld land will be substantially consumable, but provide sufficient material to negate any underfill in the base geometry. The first and second structural members are then provided having the preselected shapes and dimensions. According to one embodiment, the providing step includes forming the first and second structural members into the preselected shapes and dimensions. The forming step can include casting, forging or machining the first and second structural members. According to another embodiment, the first and second structural members comprise plates, T-stiffeners or tubular members. According to still another embodiment, the first and second structural members are formed of aluminum, an aluminum alloy, titanium or a titanium alloy. The first structural member is then positioned adjacent to the second structural member following the providing step to thereby define an interface therebetween and wherein the first and second structural members define the substantially consumable weld land. Thereafter, the first and second structural members are irradiated with a high-energy source along the interface to thereby substantially consume the weld land and join the first and second structural members to form a structural assembly. Preferably, the first high-energy source comprises an electron beam or a laser.
In still another embodiment, the structural assembly is irradiated along the interface with a second high-energy source after the first irradiating step to remove any stress concentration details and thereby provide a finished surface having improved fatigue characteristics. Preferably, the second high-energy source comprises a laser. The structural assembly can then be secured to other structural assemblies to form the frame of an aircraft.
The present invention also provides a structural assembly including a first structural member defining a first surface and a second structural member positioned adjacent the first structural member. In one embodiment, the first and second structural members comprise plates, T-stiffeners or tubular members. In another embodiment, the first and second structural members are formed of aluminum, an aluminum alloy, titanium or a titanium alloy. The second structural member defines a first surface corresponding to and substantially planar with the first surface of the first structural member. The structural assembly includes a weld joint which joins the first and second structural members. Advantageously, the weld joint is formed by irradiating the first and second structural members with a high-energy source such that a surface of the weld joint is substantially planar with the first surfaces of the first and second structural members and such that no further processing of the weld joint is necessary to create the substantially planar surface of the weld joint. In another embodiment, the first and second structural members each define corresponding second surfaces. According to this embodiment, the weld joint defines a second surface corresponding to and substantially planar with the second surfaces of the first and second structural members.
Accordingly, there has been provided a structural assembly and an associated method of manufacture allowing for the efficient construction of aircraft structural assemblies, which requires less stock material and takes less time to manufacture and assemble. The resultant assemblies include an autogenous weld having high mechanical strength and structural rigidity